1. Field of the Invention
This invention relates to a combustor of a gas turbine, and a combustion control method for a gas turbine, the combustor and the combustion control method being designed to be capable of ensuring stable combustion and suppressing the occurrence of unburned fuel.
More particularly, the combustor and the combustion control method are designed to improve flame holding properties, and suppress the occurrence of unburned fuel, during an operation in which the amount of fuel supplied to the combustor is small, as in a speed increasing state or in a low load state.
2. Description of the Related Art
A gas turbine used in power generation, etc. is composed of a compressor, a combustor, and a turbine as main members. The gas turbine often has a plurality of combustors, and mixes air, which is compressed by the compressor, with fuel supplied to the combustors, and burns the mixture in each combustor to generate a high temperature combustion gas. This high temperature combustion gas is supplied to the turbine to drive the turbine rotationally.
An example of the combustor of a conventional gas turbine will be described with reference to FIG. 10.
As shown in FIG. 10, a plurality of combustors 10 of this gas turbine are arranged annularly in a combustor casing 11 (only one combustor is shown in FIG. 10). The combustor casing 11 and a gas turbine casing 12 are full of compressed air to form a casing 13.
Air, which has been compressed by a compressor, is introduced into this casing 13. The introduced compressed air enters the interior of the combustor 10 through an air inlet 14 provided in an upstream portion of the combustor 10. In the interior of an inner tube 15 of the combustor 10, fuel supplied from fuel nozzles 16 and compressed air are mixed and burned. A combustion gas produced by combustion is passed through a transition pipe 17, and supplied toward a turbine room to rotate a turbine rotor.
In recent years, environmental restrictions have been tightened, and various improvements have been made to decrease the concentration of NOx (nitrogen oxides) in an exhaust gas from a gas turbine. The so-called rich-lean combustion process is known as a gas turbine combustion technology intended to curtail the occurrence of NOx.
With the rich-lean combustion process, combustion in a fuel-rich state (i.e., rich state) is performed in a first combustion region (in the example of FIG. 10, for example, the region in a middle portion of the internal space of the inner tube 15), while combustion in a fuel-lean state (i.e., lean state) is performed in a second combustion region (in the example of FIG. 10, for example, the region in a peripheral edge portion of the internal space of the inner tube 15). In this case, the fuel-air ratio in the entire combustor (the total fuel-air ratio which is the average of the rich state and the lean state) is controlled to a value commensurate with the operating state of (load on) the gas turbine. When such rich-lean combustion is carried out, completely diffusive combustion (combustion in the complete absence of premixing with air) takes place in the rich combustion region, and completely premixed combustion takes place in the lean combustion region.
Another example of the rich-lean combustion process is disclosed in Japanese Unexamined Patent Publication No. 1993-195822. The technology shown in this publication is based on a combustor of a gas turbine having a plurality of fuel nozzles. The fuel nozzles are divided into a plurality of groups, and the flow rate of fuel supplied to each group is controlled individually. In a low-load operating state, fuel is supplied only to the fuel nozzles of some of the groups. By this means, low-NOx premixed combustion can be performed up to a generally low load range. Also, the occurrence of unburned fuel in a low load state is prevented, and improvement in flame holding properties is achieved.
As is generally known, the relationship between the fuel-air ratio (equivalence ratio) and the amount of NOx generated is as shown in FIG. 11. At an equivalence ratio φ of about 1, a large amount of NOx is generated, and the amount of NOx generated is small in the lean region (φ<1) or the rich region (φ>1). According to the rich-lean combustion process, the amount of NOx generated can be decreased as a whole, because combustion is performed in the lean region (φ<1) where the amount of NOx generated is small, and in the rich region (φ>1) where the amount of NOx generated is similarly small.
Japanese Unexamined Patent Publication Nos. 1996-261465, 1999-14055, and 1995-12340 are other examples of related art documents.
The inventor is developing a gas turbine at a high pressure ratio (a pressure ratio of 25 or higher) in the 1,700° C. class. With the gas turbine having such a high pressure ratio, the temperature of air flowing into the combustor is very high (500 to 600° C.), and the temperature of a combustion gas supplied from the combustor into the turbine reaches 1,700° C.
With a conventional gas turbine, the pressure ratio is of the order of 20 to 21, and the temperature of air flowing into the combustor is of the order of 450° C., so that the temperature of a combustion gas supplied from the combustor into the turbine is of the order of 1,500° C.
In the gas turbine, the amount of fuel supplied to the combustor is varied according to load. That is, as shown in FIG. 4(e), when load is increased, the amount of fuel supplied to the combustor is increased. Conversely, the amount of fuel supplied to the combustor is small in a speed increasing state (during a period from the stoppage of the gas turbine until the time when the speed is increased to a rated speed) or in a low load state.
With the gas turbine at a high pressure ratio (a pressure ratio of 25 or higher) in the 1,700° C. class, the capacity of the combustor is high. Thus, if the amount of fuel supplied is decreased in the speed increasing state or in the low load state, the concentration of a fuel gas (a mixture of fuel and air) is too low, meaning an excessively lean state. If the fuel gas is too lean, the flame holding properties may deteriorate, and unburned fuel may occur in a large amount, causing a decrease in the combustion efficiency.
The present invention has been accomplished in light of the above-described problems with the earlier technologies. An object of the invention is to provide a combustor of a gas turbine and a combustion control method for a gas turbine which can ensure stable combustion and suppress the occurrence of unburned fuel even in a gas turbine at a high pressure ratio currently under development.